Chronic hepatitis C is a major public health problem and one of the leading worldwide causes of chronic liver disease, cirrhosis and hepatocellular carcinoma (1). Approximately 4 million Americans are chronically infected with HCV and as many as 25% of them may eventually develop cirrhosis (2). End-stage liver disease from hepatitis C is now the leading indication for orthotopic liver transplantation in the United States. HCV was identified in 1989 and demonstrated to be the major cause of what was then referred to as non-A, non-B hepatitis (3, 4).
The hepatitis C virus (HCV) is a positive single stranded RNA virus and a member of the Flavivaridae family (3, 6–10). Once hepatitis C virus infects cells, the positive, single-stranded RNA genome is translated into a polyprotein of 3010 to 3033 amino acids, depending upon the strain (6–9). The viral RNA is not capped and translation occurs via internal ribosome entry sites (10, 11). The mechanism of translation from uncapped viral RNA therefore differs from that used by virtually all cellular mRNAs which are capped at their 5′ ends.
In hepatocytes, the HCV core protein is mostly localized to endoplasmic reticulum membrane with a large domain facing the cytoplasm (12). It has been shown to form multimers (13). The function of HCV core protein in cells is not clear, however, it may play a role in transformation and oncogenesis (14). Such a function could hypothetically arise as a result of interactions with cellular proteins involved in signal transduction or oncogene or tumor suppressor gene products or by affecting expression from their genes. HCV core protein may also be involved in regulating the immune response as it has been shown to bind to the cytoplasmic domain of lymphotoxin-βreceptor (15). Some investigators have also shown that a truncated portion of HCV core protein can reach the nucleus (16, 17), suggesting that it may directly affect the expression of cellular genes as demonstrated in vitro (18). It is not clear, however, if this nuclear form is generated in infected cells.
The HCV polyprotein is proteolytically processed by both host cell and viral proteases into several smaller polypeptides (6–9, 12) (FIG. 1). The major structural proteins are a core protein and two envelope proteins (E1 and E2). Four major non-structural proteins called NS2, NS3, NS4, and NS5, are also generated, two of which, NS4 and NS5, are further processed into smaller polypeptides called NS4A, NS4B, NS5A, and NS5B. The non-structural proteins have various enzymatic activities, such as RNA helicase (NS3), protease (NS2, NS3–NS4A complex) and RNA polymerase (NS5B). NS5A has been implicated in determining sensitivity to interferon.
After cells are infected with a virus, viral proteins can interact with host cell proteins and influence cell physiology. In previous studies, HCV core protein has been shown to bind to lymphotoxin-β receptor and other tumor necrosis factor receptor family members (15, 27). A truncated form of HCV core protein also interacts with ribonucleoprotein K in the nucleus (28). We now show that HCV core protein binds to a cellular RNA helicase and, in experimental systems, inhibits capped RNA translation. This provides a novel mechanism by which HCV may inhibit mRNA translation in infected cells or recruit a cellular protein to enhance its own replication.
Despite major advances in diagnosing chronic hepatitis C and screening the blood supply since that time, almost nothing is known about how the virus infects, kills or transforms cells. For this reason, current therapeutic options are limited and new agents have been difficult to develop.
According to a recent National Institutes of Health Consensus Development Conference Panel Statement on the Management of Hepatitis C (5), there is an urgent need for effective antiviral therapeutics capable of inhibiting HCV replication and stopping or delaying the progression of liver disease. The Panel also concluded that a major bottleneck to the drug discovery process is the absence of a readily available cell culture system that is fully permissive for viral replication. A small animal model of HCV infection is also lacking. For these reasons, novel, alternative approaches must be developed to identify targets for the design of therapeutic agents for the treatment of patients with chronic hepatitis C.
The development of specific drugs against HCV has been impeded because there is no non-primate animal model of infection and all attempts to culture the virus have failed. Currently, the only currently approved drugs in the United States are preparations of interferon-alpha and ribavirin. The long-term cure rate of subjects treated with interferon-alpha is less than 10%. The use of ribavirin, in combination with interferon-alpha, has shown slightly better long-term cure, however, still in only a minority of subjects.